1. Field of the Invention
The present invention relates to a mask with alternating scattering bars. More particularly, the present invention relates to a low duty ratio mask.
2. Description of the Prior Art
In the semiconductor industry, the lithography process is a very important and common fabrication process. Structures of MOS devices such as layout patterns and doping regions of thin films are defined by the lithography process.
Scattering bars are employed in the lithography process to reduce or eliminate proximity effects. Rectangular openings are disposed next to edges of masked layout patterns. During an exposure process, diffraction arises at the rectangular openings and eliminates the proximity effects of adjacent layout patterns.
Resolution of the lithography process determines a line width in the semiconductor manufacture. How to increase the resolution is an important issue for the semiconductor industry. Duty ratio for a patterning is one of the features which affects the resolution. The duty ratio represents the ratio of a line width and a pitch width in a semiconductor device being fabricated. In the lithography process, a duty ratio of 1 gives the best resolution.
When the integration is further increased, for example, contact holes of a giga-bit DRAM cell, the duty ratio is less than 1 (that is, the pitch width is larger than the line width). Such patterning has a poor resolution even when the scattering bars are employed.
FIG. 1 A illustrates an exposed pattern of contact holes in a DRAM cell employing a conventional mask. Circular patterns represent positions of the contact holes. It is noted that some of the adjacent circular patterns overlap along their edges.
FIG. 1B illustrates an exposed pattern of FIG. 1A after a 0.3 microns defocus. Overlapping of edges is more serious, and hence, the masking process cannot be employed.
FIG. 2A illustrates an exposed pattern of contact holes in a DRAM cell employing a conventional mask with scattering bars. FIG. 2B illustrates an exposed pattern of FIG. 2A after a 0.3 microns defocus. Circular patterns represent positions of the contact holes. Rectangular bars indicate positions of the scattering bars. Overlapping of edges still exists. Therefore, the resolution is not improved even by employing the scattering bars.